Sulfur is a component of several amino acids in corn so it is critical for protein synthesis. Sulfur deficiencies are typically seen in soils of sandy texture, under cold temperatures, in soils with very low organic matter levels, or during periods of significant rainfall as the sulfate ion is very mobile in soils. Sulfur is also important to prevent imbalances within the plant, particularly when large amounts of nitrogen (>100 lb N per acre) are provided in a single application. Some believe that stricter air quality regulations have decreased the amount of sulfur deposited with rain and thus, sulfur deficiencies could become more prevalent.

Sulfur is better utilized by the corn plant when it is applied early in the season. Delayed applications under sulfur-deficient conditions could significantly impact yields. Ammonium sulfate (21-0-0 + 24 S) is a very good fertilizer, with lower volatilization risk when compared to urea. Also, the nitrogen in AS is readily available, and plants may benefit from a mix of ammonium and nitrate. The acidity caused by AS requires more lime to be neutralized, when compared to urea, which can help to decrease the alkalinity of some soils or increase the acidity of others. Under most conditions, applications of 100 lb AS/acre (24 lbs of sulfur in the sulfate form) are enough to meet sulfur needs. Applications of elemental sulfur are not recommended as the sulfur needs to be converted (oxidized) to sulfate which is the form the plant can use. This conversion could take several months, as it depends on temperature and moisture, thus thesulfur may notbecome available until past the time of highest needs. Sulfur is considered a non-mobile nutrient, meaning that when a deficiency is present, symptoms will show on young leaves. Nitrogen deficiency symptoms, on the other hand, will show on older leaves, as nitrogen is mobile within the plant. The pictures below show corn plants with typical sulfur and zinc deficiency symptoms.

Sulfur deficiency in corn

Zinc deficiency in corn

Farmers and consultants should be careful as a sulfur deficiency is sometimes confused with a zinc deficiency.

A typical sulfur deficiency symptom in corn appears as chlorosis (yellowing) in the leaves, but the veins remain green. A zinc deficiency, on the other hand, appears as yellow- white wide bands that begin at the base of the leaf. The margin of the leaves, the midrib, and the tip remain green in zinc deficient corn. Current recommendations include the broadcast application of zinc at a rate of 10 lb/a when the soil pH is above 6.0 and if the zinc levels are below the “optimum” level, as defined by the U of Arkansas Division of Agriculture. For soils with pH below 6.0, zinc is only recommended when the Zn levels are classified as “very low” (<1.6 ppm). Zinc sulfate is perhaps the most common source, requiring 30-33 lb/a to supply 10 lb/a of the zinc nutrient. Research has shown that the source of zinc needs to be at least 50% water soluble to effectively deliver the amount of zinc needed. Research with corn in Colorado showed that banding zinc at a rate of 2 lb/a delivered enough zincto supply the crop needs. Foliar applications of zinc in corn, under deficient conditions, have had mixed results in other states particularly due to low uptake efficiency in young corn plants. Enough zinc should be applied to provide 0.5 lb/acre, with a wetting agent recommended to increase uptake efficiency. Zinc applications of 1 lb/a or more are discouraged as leaf burn is possible.